Manual Unwrap Warping Seams
For this block-shaped object just simply use cube uv projection and re-arange the pieces  like you want in uv editor.

For more complex shapes unwrap the planar part with project from view so they don't distort, pin them in uv editor with P and then unwrap the whole piece with just Unwrap (with setuped seams).

For multiple planar parts in your mesh do this multiple times to create uv islands each with undistorted pinned piece inside and then use welding (W+W) to join the islands. For best results smooth the welds with UV Sculpt's (Q) Relax brush.